This invention relates generally to disc-shaped recording media, and, more particularly, to direct-read-after-write (DRAW) optical recording discs.
Optical recording discs of this particular type typically include a pair of thin, transparent disc-shaped members arranged in a spaced, coaxial relationship, with a thin layer of a special light-sensitive material overlaying the inwardly-facing side of at least one of the two members. The light-sensitive recording layer is normally located within an annular chamber, to protect it from any adverse environmental conditions that might be present and, also, from possible damage due to manual handling. The light-sensitive material normally comprises a material that exhibits a change in its optical characteristics, such as melting or evaporating to produce a microscopic hole, whenever a beam of light of sufficient intensity, is focused thereon. Suitable materials include, for example, metals such as bismuth or titanium, semiconductors such as tellurium, selenium, or arsenic selinide, or dielectric materials such as silicon dioxide in combination with a suitable dye.
An information signal is recorded in the optical disc by focusing onto the light-sensitive recording layer a beam of light, modulated in intensity in accordance with the information signal, as the disc is rotated in a prescribed fashion. The intensity of the beam is alternately greater than and less than a predetermined threshold, at which melting or evaporation of the light-sensitive layer occurs, whereby a sequence of spaced holes, representative of the information signal, is formed in the layer, in a succession of substantially circular and concentrically arranged recording tracks. The recorded disc can then be read immediately, without any intermediate processing of the disc.
One example of a DRAW optical recording disc is disclosed in U.S. Pat. No. 4,074,282, issued in the name of C. Balas, Jr. et al and entitled "Radiation-Sensitive Record with Protected Sensitive Surface". The record includes a pair of planar, disc-shaped members that are spaced from each other by a pair of separate, ring-shaped spacers or seals, one located adjacent the outer peripheries of the two members and the other located near their respective centers. A planar, annular chamber is therefore formed between the two disc-shaped members, and a light-sensitive layer is coated on the inwardly-facing surface of each member, within the chamber.
Although the aforedescribed disc-shaped record has proven generally effective as a record medium capable of withstanding adverse environmental conditions and handling, it is felt that the record includes more elements than are necessary to provide equally satisfactory performance. Moreover, difficulties can sometimes arise in aligning the two disc-shaped members along with the two separate, ring-shaped seals, in a precise coaxial relationship, during assembly of the record. Additionally, difficulties can arise in determining the precise annular portions of each disc-shaped member, on which the light-sensitive material is to be coated.
It will therefore be appreciated that there is a need for a disc-shaped optical recording medium in which a light-sensitive layer for recording information is adequately protected from adverse environmental conditions and from handling, without requiring an excessive number of elements, and without presenting difficulties during its assembly. The present invention fulfills this need.